"We have determined that Internet Explorer was one of the vectors used in targeted and sophisticated attacks against Google and possibly other corporate networks," said Mike Reavey, director of Microsoft's Security Response Center (MSRC), in a post to the group's blog.

Earlier today, antivirus company McAfee said the IE bug had been exploited by hackers who had attacked computer networks of nearly three dozen major companies between mid-December 2009 and Jan. 4, 2010. McAfee said then that Microsoft would soon release this advisory.

The security advisory said that the only version of IE not containing the critical flaw was IE 5.01 running on Windows 2000. All other versions, including IE6, IE7 and IE8 on Windows 2000, XP, Server 2003, Vista, Server 2008, Windows 7 and Server 2008 R2 are vulnerable to attack.

Even so, Reavey downplayed the threat to average Windows users. "Microsoft has not seen widespread customer impact, rather only targeted and limited attacks exploiting IE6 at this time," he said.

"An IE zero-day in all versions," said Andrew Storms, director of security operations at nCircle Network Security, "so by no means is this good for Microsoft. The only encouraging news is that there are tools that protect Vista and Windows 7 on IE7 and newer, so that an exploit would crash [those browsers] rather than allow code execution." Storms was referring to security provisions within IE, including DEP (data execution prevention) and Protected Mode, on newer versions of Windows.

Microsoft's Reavey hammered that home as well. "Protected Mode in IE 7 on Windows Vista and later significantly reduces the ability of an attacker to impact data on a user's machine," Reavey said. "Customers should also enable Data Execution Prevention, which helps mitigate online attacks."

Although DEP is on by default in IE8, it must be manually switched on in IE6 and IE7. Users can enable DEP by using the "Fix it" tool Microsoft has posted on its support site.

As McAfee noted earlier today that an IE user's PC could be hijacked simply by steering the browser to a malicious site, or to a compromised legitimate site that hosted attack code.

Microsoft said users could also protect themselves to some degree by setting the PC's Internet zone's security to the "High" option, but warned that it wasn't surefire. "It is important to note that the vulnerable code may be reached even with these protections in place," the company said in the advisory. "However, any attacks would be less successful with these workarounds in place."

The company did not set a timetable for producing patch, but Storms was certain that Microsoft would scramble to get something out as soon as possible. "For sure they'll do an out-of-band update," said Storms, using the term for a security fix that's released outside Microsoft's monthly patch schedule. "The public relations aspects are going to drive this."

Storms was talking about the criticism that Microsoft is sure to harvest as the vendor whose software let hackers break into dozens of major Western companies.

Microsoft last issued an out-of-band update in late July 2009, when, ironically, it patched IE just hours before several researchers demonstrated the vulnerability at a security conference.

The attacks first came to light Tuesday, when Google announced that Chinese attackers had made off with intellectual property from its corporate network, and also tried to access the Gmail accounts of Chinese human rights activists.

McAfee today scotched such talk, saying that although it did not investigate every attack, it had worked with several targeted corporations and found evidence of only one vulnerability: the IE zero-day.